Based on the serial #, I'd say it was made in 1979.
http://www.guildguitars.com/support/dating.php
If the guitar were in good condition, I'd say it would be worth much more. There are a couple of G312s on harmony central for which the buyers paid US$685 and US$900 used.
For reference, a new Guild G212 goes for a little over $700 - prices have come down over the years. The 212 is probably a comparable model to your friend's 312, and the 212 is their least expensive all-solid 12 stringer.
However, a crack in the neck and bridge lift can be the start of serious problems on a 12-string, with all the extra string tension. Take it to a good luthier and get a repair estimate. That's the only way you'll know.